The invention relates generally to a sheet gripper for a sheet-processing machine and, more particularly, to a sheet gripper having a swingable gripper arm which has at one of its ends a gripper tip that can be pressed against an abutment and, at its other end, a spring section which is bent back towards the gripper tip in the manner of a bent leaf spring and connects the gripper arm to a gripper shaft.
In sheet-fed printing machines the sheets to be printed are transferred from one cylinder to the next by means of sheet grippers in order to transport the sheets from printing unit to printing unit. It is extremely important that satisfactory transfer of the sheets takes place from cylinder-to-cylinder. Only in this way can a good printing quality be insured. For this purpose, the grippers must impose a sufficiently large holding force which amounts to about 10 kp per gripper. The grippers should be as light as possible and not extend a significant distance from the axis of rotation of the gripper shaft so that the mass moment of inertia is not too great. Furthermore, the grippers should be able to be exchanged easily since they are subject to wear, and they should be adjustable in terms of their holding force.
A sheet gripper of the type specified is disclosed in Belluche U.S. Pat. No. 2,395,444. Such a sheet gripper comprises a continuous steel strip of uniform cross-section forming a gripper arm, a bent-back spring region and a circular arc-shaped clip for attachment of the spring region to the gripper shaft. The clip encloses a circumferential angle of nearly 360.degree.. In the Belluche sheet gripper, the bent spring region is located between the gripper shaft and the outer contour of the impression cylinder. To keep the spring region from projecting beyond the outer contour, the gripper shaft must be arranged a corresponding distance inwardly from the outer contour of the impression cylinder. This results in the gripper tip moving in non-perpendicular relation to the outer surface of the impression cylinder. Instead, the gripper tip moves at an angle relative to the outer surface of the impression cylinder. As a result, the gripper tip can still move in the circumferential direction when it rests on the outer surface of the impression cylinder and is turned even further when the holding force is increased. Furthermore, adjustment of the gripper arm by the clip is difficult, and exchange of the sheet grippers requires the gripper shaft to be dismantled.
A sheet gripper also is disclosed by DE-A-3,623, 405 and comprises a plurality of individual parts, namely, a gripper arm, a carrier, an attachment screw, an adjustment screw and a helical spring. The carrier is screwed to the gripper shaft and, like the gripper arm, extends tangentially relative to the gripper shaft. With the carrier firmly connected to the gripper shaft, the gripper arm can move in a rotating and tilting manner relative to the gripper shaft. The helical spring is located between the carrier and the gripper arm and is prestressed by means of the adjustment screw in order to establish a holding force determined by the setting of the adjustment screw. This arrangement is disadvantageous in that it consists of a plurality of individual parts and is therefore complex in manufacture and difficult to exchange.
Another sheet gripper, likewise consisting of a plurality of parts, is disclosed in German Utility Model 8,404,981. Attached to a basic body, which is firmly connected to the gripper shaft, is a leaf spring which forms the gripper arm. The leaf spring projects beyond the basic body and includes a tip which forms the gripper tip. The leaf spring rests on an adjusting screw which is screwed into the basic body. By adjusting the adjusting screw relative to the basic body, the prestress of the leaf spring can be varied. The basic body consists of a block-like part which rests on one side of a spindle and is held by a strap which engages around the spindle. In this arrangement, the angular position of the gripper tip disadvantageously changes depending on the deflection of the leaf spring.